1. Field of the Invention
The present invention relates to a control function for preventing dimensional error in machining by numerically controlled machine tools equipped with an automatic tool exchanger and, more particularly, to measurement of run-out of the outer surface of a tool holder in order to determine whether the tool has been properly mounted on the spindle of a numerically controlled machine tool.
2. Description of the Prior Art
In machine tools, high machining accuracy, low machining failure rates, and non-delivery of poorly machined products are demanded. For machine tools equipped with an automatic tool exchanger, there is a particularly strong need for detecting chips caught between the tool holder and the spindle during operation of the automatic tool exchanger, to prevent the machining defects that such chips can cause.
Chips caught between a tool holder and a spindle cause a tool holder run-out, resulting in inaccurately machined dimensions. Techniques for determining run-out due to chips are disclosed in, for example, the following references.
Japanese Patent Application Laid-open No. 2002-200542 discloses a machine tool which measures run-out of a flange section of a tool holder including key grooves by using a displacement detector while rotating the spindle, performs a Fourier analysis of measured data, and calculates the eccentricity of the tool holder from the amplitude of the fundamental frequency component.
Japanese Patent Application Laid-open No. 2003-334742 discloses a displacement detector which measures run-out of a flange section of a tool holder. This displacement detector is made movable in forward and backward directions so that the sensitivity of the displacement detector is corrected.
Japanese Patent Application Laid-open No. 2001-347440 discloses a machine tool which rotates a spindle to which a test tool is attached, detects the displacement of the outer surface of the test tool by using a displacement detector, and compares the detected displacement with a tolerance in order to diagnose the run-out accuracy of the spindle.
To transmit the rotation of a spindle to a tool, a tool holder usually has a key groove on its cylindrical outer surface to accept a key on the spindle. In the machine tools disclosed in the prior art, which use a displacement detector to measure the distance to the outer cylindrical surface of a tool holder, the displacement detector measures a portion of the key groove in the state where the displacement detector is located away from the key groove. Therefore, an accurate measurement cannot be made just by turning the spindle, because it cannot be determined whether measured variations are attributable to poor tool attachment due to chips or other foreign objects caught between the tool holder and spindle, or due to the portion of the key groove.